This invention relates to a method and apparatus for detecting defects on the interior of a tube or the individual tubes of tube bundles widely utilized in petroleum refineries, condensers, steam boilers and the like to determine the exact location and degree of localized tube interior deterioration that has occurred through erosion, corrosion, pitting and other causes for material on the interior surface of the individual tubes being displaced and eaten away.
The tubes in boilers, oil refineries and the like are subjected to high pressure exceeding 1,000 pounds per square inch. The flow of liquids, which are often corrosive in nature, through the tubes causes localized or generalized pitting and thinning of the tube walls. Therefore, as a safety measure non-destructive tests of the tubes should be made at intervals to determine if there are areas in the walls of any of the tubes which have been weakened by erosion, corrosion or pitting to the point where these weakened sections of the tubes should be cut out and replaced or the entire tube replaced. Up to this time there have been few satisfactory non-destructive methods of testing tubes, particularly in testing tubes on the interior of tube bundles. Eddy current testing, through introducing a probe into the tube to induce an electro-magnetic field in the sections of the tube through which the probe traverses, can be used for non-ferrous tubes but is not satisfactory for ferrous tubes which become magnetized by the induced field of the probe which cancels out the test results. A technique utilizing a D.C. saturation coil in connection with the probe to reduce the magnetic permeability of the ferrous tube to zero has been partially successful but the inherent problems have kept this new testing procedure from being generally accepted. Ultrasonic testing has also been utilized in which the tubes are traversed by a piezo electric crystal. However, water must be used as a coupler between the piezo crystal and the tube wall, and water is unacceptable in some tube environments such as those which are exposed to acidic conditions. Further, this method is quite time consuming.
The present invention utilizes some of the basic principles of pneumatic gauging devices, sometimes known as air micrometers, which have been commonly utilized in many forms to determine the internal dimensions of hollow objects. Such devices include a probe having fixed orifices and connected to a source of air pressure. The probe is inserted into a workpiece having unknown internal dimensions and the back pressure created by the air flow through the probe is compared to the back pressure created by the probe in a bore of known dimensions to provide an indication of the unknown internal dimensions. Typical of such pneumatic gauging apparatus are the devices disclosed in U.S. Pat. Nos. 3,438,224; 4,088,009 and 4,125,011. However, these devices are designed to determine the average internal dimensions of the hollow object being measured and do not detect small localized areas of pitting. The problem faced by the applicant is that of detecting, accurately locating and measuring the depth of small localized areas of deterioration, of which pitting is the most serious. Although the prior art devices might detect the overall thinning of a major segment of the tube wall due to a washing type of displacement of tube wall material along a significant portion of the tube, these prior art gauging devices will not detect, locate and measure the depth of small localized areas of pitting. Localized pitting is much more serious than a generalized reduction of the tube wall thickness. A tube having the wall thickness reduced to a given amount in a small localized area from pitting will blow out whereas a tube having its overall wall area thinned to the same amount will not. Therefore, the detection and location of localized areas of tube interior pitting is of maximum importance.